1. Field Of The Invention
This invention relates to a measuring system that utilizes a variety of velocity informations, more particularly to an ultrasonic measuring system for automotive use. The measuring system may be applied to a navigation system, speed detecting system, side slip preventing system, antilock brake system, suspension system, centrifugal force detecting system, yaw velocity or yaw rate detecting system, and the like.
2. Description Of The Related Art
Japanese Published (Kokai) Utility Model Application No. 57-68574 discloses an ultrasonic speed measuring system. In this technique, a transmitter is separately provided to successively send ultrasonic waves. The waves are reflected on a reflector and successively go into a receiver. Then, Doppler frequency is measured by a difference between the transmitted waves and the received waves. This technique is well-known to those skilled in the art.
Japanese Published (Kokai) Patent Application No. 59-203973 discloses another ultrasonic speed measuring system. In this technique, one transmitter and two receivers are separately arranged as in the above mentioned technique. Particularly, these two receivers alleviate error due to pitching, nose-up or nose-down of a car body.
Japanese Published (Kokai) Patent Application No. 58-39971 also discloses an ultrasonic speed measuring system. In this technique, ultrasonic waves are transmitted in pulse form. A receiving gate corresponding to pulse width is opened when the reflected waves are received. Then, time period of a predetermined number of received waves is measured to obtain an amount of Doppler shift, thereby metering a car speed.
Japanese Published (Kokai) Patent Application No. 3-269388 discloses still another ultrasonic speed measuring system.
In this technique, a transducer radiates ultrasonic waves toward a road surface in front of or at the back of a car at a pre-defined down-angle therefrom. It is measured how long the wave takes to travel from the transducer to a protrusion of the road, on the basis of the radiated waves and input signals of the waves reflected from the protrusion. Moreover, the signal level of the reflected waves is compared with a predetermined threshold so as to detect presence of the protrusion or the like on the road in front of the car and its size. Furthermore, a current car height is measured on the basis of a radiating angle of the waves and a linear distance in the time when the reflected waves come back from the road surface thereby measuring a current speed on the basis of thus obtained Doppler frequency.
FIG. 9 is an explanatory drawing illustrating a fundamental theory of an operation of the ultrasonic measuring system that uses one ultrasonic transducer TR for transmitting and receiving the waves. In the figure: EQU L=H/ sin.phi. (1)
where: L[m] is a distance from the transducer to the road surface, H[m] is a height of the transducer, and .phi. [degree] is a radiating angle of the waves from the transducer. PA1 .alpha.100 KHz=2.1 [dB/m] PA1 .alpha.200 KHz=8.5 [dB/m] PA1 V is a car speed [Km/h], and PA1 C is a sound velocity [m/s].
Loss LOSS attributable to a propagation distance at that time is: ##EQU1## where: .alpha. is an attenuation constant.
For instance:
In case ultrasonic wave beam width (.theta. degree) is narrowed, in the transmitted waves, their energy is more intensive thereby increasing signal components S. In the received waves, their signal-to-noise ratio (S/N) is improved for isotropic noises.
Gain G of the transmitted and received waves in total is: ##EQU2## where: .gamma.=3.4.times.10.sup.4 (in case the beam is rotationally symmetric.)
In FIG. 9, if ultrasonic waves of a frequency f [Hz] are intermittently radiated to the road surface and Doppler frequency df [Hz] is calculated from the received wave frequency f0 (f0=f-df[Hz]): EQU df=2f(V/3.6) cos .phi./C[Hz](4)
where:
Specifically, in the above technique, the ultrasonic waves are radiated from the car toward the frontward direction and the rearward direction at the same radiation angle, and the Doppler frequency of the received signals of the reflected waves of each direction is measured. Then, a Doppler frequency of a difference between the Doppler frequencies of the frontward waves and the rearward waves is calculated so as to obtain a car speed in which a vertical speed component of a car body is canceled. A current car height is obtained from a measurement of the vertical speed component.
Thus, the measuring system detects the protrusion of the road in front of the vehicle at the time of running, utilizing the ultrasonic waves, and measures the car height and the car speed.
As described above, one of the conventional systems obtains the car height on the basis of the linear distance in the time of the reflected waves returning from the road surface and the radiation angle of the waves, thereafter calculating the car speed on the basis of thus obtained Doppler frequency. Another conventional system radiates the ultrasonic waves ahead and behind of the car at the same radiation angle so as to detect the Doppler frequencies, respectively, thereafter calculating the Doppler frequency of their differences and obtaining the car speed in which the vertical component of the car body is canceled.
However, such ultrasonic measuring apparatuses which measure the car movement using the ultrasonic waves have limited uses such as measurements for the car speed, car height and the like while they measure the speed component.